Organophosphate (OPs) insecticides have become the most widely used insecticides available today. OPs are used in agriculture, at home, in gardens, and in veterinary practice. Since most of these compounds inhibit some esterase enzymes, exposure to OPs can lead to serious toxicity by multiple routes. Irreversible inhibition of acetylcholinesterase by OPs, a key enzyme of the mammalian nervous system, causes severe damage for all vertebrates. Loss of enzyme function leads to accumulation of acetylcholine in different compartments of the body causing muscle contraction, paralysis and respiratory depression. Increased pulmonary secretions with respiratory failure are the usual causes of death from organophosphate poisoning.
Some of OPs have also been developed by armies before the World War II. The discovery of OPs with improved toxicity and/or higher stability has led to the development of chemical warfar agents (CWA) such as sarin, soman, tabun or VX. Moreover, OPs insecticides, being easily accessible and not so less toxic as compared to CWA OPs, constitute an important risk for the population. Faced with these growing threats, the development of anti-dotes has never been more urgent.
OPs are efficiently absorbed by inhalation, ingestion, and skin penetration because of the hydrophobicity of these molecules. The occurrence of poisoning depends on the absorption rate of the compound. Symptoms of acute OPs poisoning develop during or after exposure, within minutes to hours, depending of the method of the contact. Exposure by inhalation results in the fastest appearance of toxic symptoms, followed by the gastrointestinal route and finally dermal route.
Protective suits and masks do not always offer an effective protection against OPs. In patients poisoned by OPs contamination of skin, clothing or hair, decontamination must proceed with surgical soap or laundry detergents. Treatment of highly contaminated persons results in administering atropine or diazepam which antagonizes the effects of excessive concentrations of acetylcholine at end-organs having muscarinic receptors. Unfortunately, atropine remains ineffective against nicotinic actions, specifically muscle weakness and respiratory depression in case of severe poisoning. Pralidoxime, a cholinesterase reactivator, relieves the nicotinic as well as the muscarinic effects of OPs poisoning when administering quickly after poisoning. The use of this compound remains uneffective against sarin which holds a very quickly effect once inhalated. Clearing airway and improving tissue oxygenation is also very helpful.
Although some progress in prophylaxia has been made with the abovementioned techniques, existing protection and the treatments for these poisoning nevertheless remain unsatisfactory.
The first OPs-hydrolases have been identified in several bacteria in the early 90's (Cheng et al., 1993, Appl. Environ. Microbiol., 59: 3138-3140, Raveh et al., 1993, Biochem Pharmacol., 45: 2465-2474). Butyrylcholinesterase (BChE)- and acetylcholinesterase (AChE)-based OP bioscavengers were considered as potential stoichiometric traps. Unfortunately, due to their low stoichiometric binding capacity to OPs, huge quantity of BChE or AchE is needed to cure the poisoning individuals. This renders the use of these enzymes disproportionate and quite expensive.
Some other microbial enzymes generally called phosphotriesterases (PTEs) show preferences for organophosphorous compounds with P—O or P—S bonds. These enzymes are members of the aminohydrolase superfamily, enzymes catalyzing hydrolysis of a broad range of compounds with different chemical properties (phosphoesters, esters, amides, etc). Their coding genes, opd (organo phosphate degradation), were isolated in soil bacteria such as Pseudomonas diminuta, also called Brevundominas diminuta (Munnecke et al., 1976, Appl. Environ. Microbiol., 32: 7-13), Flavobacterium sp. (Sethunathan et al., 1973, Can J Microbiol, 19: 873-875) and Agrobacterium radiobacter (Horne et al., 2003, FEMS Microbiol Lett, 222: 1-8), and genes similar to opd were also identified in Archaea (Merone et al., 2005, Extremophiles, 9: 297-305).
Lactones are signalling molecules synthesized by bacteria which allow them to detect the population density. This cell-to-cell communication process is termed quorum sensing (QS) and is well known to modulate many key biological functions of bacteria including biofilm formation (Popat et al., 2008, British Medical Bulletin, 87: 63-75). This link between QS and virulence is central to the pathogenesis of many bacterial infections, including P. aeruginosa (Sakuragi et al., 2007, J Bacteriol, 189: 5383-5386) but also A. baumanii (Stacy et al., 2012, ACS Chem Biol, 7(10): 1719-1728), Bulkolderia sp. (McKeon et al., 2011, J Infect Dis, February 1; 203(3):383-92), Vibrio sp. (Augustine et al., 2010, Arch Microbiol 192(12): 1019-1022) or E. caratovora (Dong et al., 2001, Nature, 411: 813-817). Interfering with QS system, also called quorum quenching, is a promising approach to control bacterial diseases in plants and animals (Dong et al., 2001, nature, 411: 813-817). N-acylhomoserine lactones (AHLs) are molecules that mediate bacterial communication for many Gram negative bacteria and some Archaeal organisms (Zhang et al., 2012, ISME J., July; 6(7):1336-44). It classically regulates infection and virulence functions. These molecules accumulate in the media to reach a certain threshold for which the transcriptional profile of the bacteria is altered (Hentzer et al., 2003, Embo J, 22: 3803-3815). By hydrolyzing AHLs, lactonases like PLLs can quench the AHL-mediated communication between bacteria, as seen for human paraoxonases (Ma et al., 2009, Appl Microbiol Biotechnol, 83: 135-141) or AiiA lactonase (Dong et al., 2001, Nature, 411: 813-817). Because of their dual catalytic activities, lactonases and phosphotriesterases, PLLs constitute highly attractive candidate for biotechnological utilization as quorum quenching agent or OPs bioscavenger.
Recently, new thermophilic bacteria belonging to the phylum of crenarchaeota have been discovered in Japan and Russia. Vulcanisaeta species were found in hot springs of volcano area. The complete genome of two currently known Vulcanisaeta species, V. distribute and V. moutnovskia was sequenced aiming to understand the physiological properties of this archaeon and their possible ecological roles (Mavromatis et al., 2010, Stand Genomic Sci.; 3(2):117-25/Gumerov et al., 2011, J Bacteriol.; 193(9):2355-6). Among the protein-coding genes, few sequences share similarities with the ones encoding hyperthermophilic PTEs or Sulfolobal PLL lactonases, assuming some similar enzymatic activities.
Phylogenetics and sequence alignment of PTEs genes with V. moutnovskia genes show a percentage of identity close to 30%. Due to the low sequence of identity with known OP hydrolases and lactonases (maximum 50%) different substrate specificities and catalytic properties are highly expected.